This invention relates generally to electromagnetic flowmeters, and more particularly to a technique for injection-molding an insulating liner onto the inner surface of a metal spool to define a flow conduit for the fluid being metered.
Magnetic flowmeters such as those disclosed in U.S. Pat. Nos. 3,695,104; 3,824,856; 3,783,687 and 3,965,783 are especially adapted to measure the volumetric flow rates of fluids which present difficult handling problems, such as corrosive acids, sewage and slurries. Because the instrument is free of flow obstructions, it does not tend to plug or foul. The flowmeter can be used to meter liquids without regard to heterogeneous consistency.
In a magnetic flowmeter, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow tube through which the fluid to be metered is conducted and to the transverse axis along which the electrodes are located at diametrically-opposed positions with respect to the tube. The operating principles are based on Faraday's law of induction, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. The metered fluid effectively constitutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instantaneous value of the voltage established at the electrodes.
In order to provide a compact and readily installable electromagnetic flowmeter whose weight and dimensions are substantially smaller than existing types, the Schmoock U.S. Pat. Nos. 4,253,340 and 4,214,477 disclose a highly compact flowmeter which, in spite of its reduced volume and weight, is capable of withstanding high fluid pressures. In the Schmoock flowmeter, use is made of a non-magnetic metal spool of high strength whose inner surface is lined with insulating material to define a flow conduit for the fluid to be metered. The spool also serves to withstand fluid pressure as well as the compressive forces to which the meter is subjected by bolts bridging the flanged ends of upstream and downstream pipes between which the unit is interposed.
Surrounding the Schmoock spool and concentric therewith is a cylindrical housing formed of ferromagnetic material. The housing is provided with annular end plates that are joined to the corresponding end flanges of the spool to define an inner chamber. Integral with the housing are two magnet cores which are placed at diametrically-opposed positions along an axis which is normal to the longitudinal axis of the housing coils being wound on these cores. A pair of electrodes are mounted on the spool at diametrically-opposed positions along a transverse axis at right angles to the core axis. The inner chamber is filled with a potting compound to encapsulate the electromagnets and the electrodes, the housing serving as a mold for this purpose.
Insulating liner for electromagnetic flowmeters are usually molded of fluorocarbon materials such as PTFE, PFA and FEP. Because fluorocarbons are non-reactive with virtually all corrosive fluids, they have the properties appropriate to liners for flowmeters. When injection-molding plastic liners into the body of metal spools of the type included in flowmeters disclosed in the Schmoock patents, certain problems are encountered.
One problem which occurs regardless of the nature of the molding material is when the molten thermoplastic material encounters an obstruction in its flow path, such as a core pin or an insert. The molten material is then forced to separate in order to flow around the obstruction; and in that situation, a weld or knit line will be formed where the two flow fronts join on the downstream side of the obstruction in the flow path. Such weld lines create weakened areas in the molded liner. Since the liner is subjected to fluid that may be under high pressure or include abrasive contaminants, the liner in some instances will in time be disrupted in the weakened areas.
Another problem arises in conventional injection-molding techniques when use is made of thermoplastic resin molding materials which have reinforcing fibers therein, such as TEFZEL, a fluoropolymer marketed by the DuPont company.
TEFZEL is the trademark covering a family of melt-processable thermoplastics (ETFE) with an outstanding balance of properties. Mechanically, TEFZEL is exceptionally tough, having excellent flex life, impact, cut-through and abrasion resistance. The glass fiber reinforced compound (Tefzel HT 2004) has even higher tensile and compressive strength, stiffness and creep resistance. Thermally, "Tefzel" has a continuous temperature rating of 150.degree. C., the material being inert to most solvents and chemicals. It is an excellent low-loss dielectric with a uniformity of electrical properties normally absent with other thermoplastics.
The concern of the present invention is with "Tefzel" or other suitable thermoplastics having reinforcing fibers therein. In molding a fiber-reinforced thermoplastic material one must take into account fiber orientation. If the fibers in the molded flowmeter line are aligned in the direction of material flow which is parallel to the longitudinal axis of the meter, this orientation reduces material shrinkage in this direction and thereby prevents the liner from pulling away from the surface of the metal spool. But with conventional injection-molding techniques in which the liner is required to conform to the inner surface of the metal spool which is not purely cylindrical but includes shaped regions, the desired fiber orientation is not realized.
If, as is usually the case with conventional injection-molding techniques for liners, two flow fronts meet at a weld line, the fibers embedded in the molding material will lie parallel to the direction of flow. As a consequence there will be no fibers extending through the plane of the weld line. And because the plane of the weld line is then devoid of reinforcement, stresses thereafter exerted on the liner will concentrate at the weld line and result in failure.